Thomson, Philips, and Toshiba all sell the HV rated RTV for use on PTV tube anodes. It is not inexpensive, but is required per the manufacture due to the close proximity of the other wires and grounds.
The key is to have a very clean tube surface and new anode that is also clean and dry. The rtv must be allowed to fully CURE before applying high voltage. To guarantee a full cure, I would wait 2 or more days. It is also important not to over-apply the rtv. Just enough to provide a good air-tight seal around the outer 1/4" to 1/2" of the hv anode is all that is required.
Failure to properly seal the anode to the tube will result in hv arcing and premature failure of the repair.
Regular acetic acid based silicone rtv should NEVER be used. The acid eats away the metal in the hv anode connections. The silicone is not guaranteed insulative at the votlages being applied.